1. Field of the Invention
The invention relates to compositions and methods for the preparation of a photochromic polycarbonate lens. In particular, methods involving the use of a tie coating to adhere a photochromic matrix to a polycarbonate lens are disclosed.
2. Description of the Related Art
Individuals who wear prescription corrective eyeglasses have several alternatives when moving from low light conditions (e.g. indoors) to high light conditions (e.g. outdoors in sunlight). First, a second pair of prescription eyeglasses having permanently darkened lenses can be used. This has the advantage of being permanently darkened or tinted, but this necessitates the purchase and carrying of two sets of eyeglasses for comfortable vision in low light (indoor or night) and high light (outside sunlight) conditions, and the individual can easily lose or misplace whichever pair of eyeglasses is not currently in use. As a second option, permanently darkened non-corrective lenses can be attached to the corrective eyeglasses. These can be attached in a variety of ways, such as using a hinge, mechanical clips, or magnets. The non-corrective lenses must also be purchased, carried, and are at risk of being lost or misplaced. A third option is to have the corrective eyeglass lenses be photochromic, changing from transparent in low light conditions, to darkened in high light conditions.
Plastic photochromic lenses have been on the market for well over a decade. Their photochromic performance has improved as the technology has progressed. Measurement parameters reflecting photochromic performance include:                Faded transmission: a high level of visible light transmission through the lens is most desirable (when in room light or away from direct or intense reflected sunlight);        Darkened transmission: a low level of light transmission through the lens is most desirable (when exposed to sunlight);        Dynamic range: the difference in the percent of light transmission through the lens between the fully faded state and the darkened state. A wide range is most desirable;        Thermal stability: sustained low light transmission when the lens is exposed to sunlight in a hot environment (e.g. desert or tropical). Minimizing the reduction of dynamic range at higher temperatures is most desirable;        Time to darken from faded state to darkened state: a short time period is most desirable;        Time to fade from the darkened state to the faded state: a short time period is most desirable; and        The ability to fade in a reasonable amount of time when darkened in a cold environment (e.g. winter weather): a short time period is most desirable.        
Ideally, photochromic lenses exhibit high faded transmission, low darkened transmission, a wide range of transmission between faded and darkened states, rapid conversion between faded and darkened states, and good long term stability. These performance characteristics are generally expected to be maintained for at least a two year lifetime.
Polycarbonate lenses are increasingly gaining popularity with corrective eyeglass consumers. Polycarbonate lenses have attractive price, impact strength, high index of refraction, and lower weight (i.e. low density) attributes as compared to conventional glass or plastic lenses.
It is desirable to make photochromic polycarbonate lenses because of the market demand for polycarbonate lenses in general. Price, high refractive index, impact strength and low density are popular attributes desired by lens customers, and polycarbonate lens blanks are relatively inexpensive to make. Clear optical grade polycarbonate is a widely available commodity, and the injection molding process allows for a clean mass-production manufacturing operation. However, making a polycarbonate lens with photochromic properties presents a unique set of challenges. Commercially available photochromic dyes do not perform satisfactorily when mixed directly into polycarbonate thermoplastic resin. The heat and time necessary for injection molding the polycarbonate degrades most dyes. Additionally, the flexural modulus and crystallinity of the solidified thermoplastic can interfere with the dye's ability to change states. Polycarbonate does not directly bond to many materials that provide environments favorable to photochromic dye performance, and polycarbonate is severely altered by many chemical species.
Various patents have issued describing attempts to overcome various challenges posed by the production of photochromic polycarbonate lenses.
U.S. Pat. No. 5,523,030 (issued Jun. 4, 1996) describes a double injection molding process in which a thermoplastic, loaded with photochromic dye, is injection molded into a cavity. After the first shot has cooled below its glass transition temperature, a second shot of a thermoplastic (such as polycarbonate) is delivered into the enlarged cavity. Upon cooling, the lens blank is to have no distinct phase boundary. This method conserves expensive dyes. This patent did not address the lack of photochromic performance that would exist in a polycarbonate-like photochromic layer capable of leaving no phase boundary.
U.S. Pat. No. 6,367,930 (issued Apr. 9, 2002) suggests the use of a more photochromic friendly thermoplastic. In this document, the thermoplastic polyurethane containing the photochromic dye is either: placed in the mold cavity first as a film, and the cavity subsequently injected with polycarbonate; or, the cavity is first injection molded with polycarbonate, the cavity enlarged and a second injection is made with the photochromic polyurethane.
U.S. Pat. No. 5,827,614 (issued Oct. 27, 1998) and U.S. Pat. No. 6,328,446 (issued Dec. 11, 2001) concern the use of a layered film where the exterior layers are thermoplastic, and the interior layer is “functional”, being either photochromic or dichroic. These films can be placed against the mold base and followed with an injection of polycarbonate.
Coating methods are popular with many lens manufacturers because they do not require special injection molding equipment, or added operations with the mold cavities. Additionally, commonly available coating equipment could be employed in the photochromic coating application with some modification (e.g. speed, nozzle, light source). The photochromic coating could also be applied by an outside service firm.
The majority of photochromic polycarbonate lenses on the market today are made through the use of coatings applied to polycarbonate lens blanks. The coatings tend to be 10-30 microns thick. U.S. Pat. No. 6,025,026 (issued Feb. 15, 2000) describes this approach using organic anhydrides and/or isocyanates. A polymeric substrate lens is treated to provide surface reactive groups, and a polymerizable composition is applied to the surface to create an adherent polymeric layer. The polymerizable composition can contain photochromic or tinting agents.
U.S. Pat. No. 6,150,430 (issued Nov. 21, 2000) describes the use of organofunctional silanes and actinic radiation to adhere photochromic coatings onto a polymeric substrate. A polymerizable composition of a surface modifying amount of an organofunctional silane, a catalyzing amount of material which generates acid upon exposure to actinic radiation, and a solvating amount of solvent is applied to the surface of a polymer modified to contain reactive functional groups. Next, the surface is treated with actinic radiation. Finally, a photochromic or non-photochromic polymer-forming composition is applied and cured.
U.S. Patent Application No. 2002/0136899 (published Sep. 26, 2002) describes the formulation and use of a photochromic elastomeric polyurethane coating on various lens substrates, including polycarbonate. Preferred elastomers include chlorosulfonated polyethylene, fluoroelastomers, neoprene, polysulfide, and polyurethane.
A composite lens in which the outer layer, containing the photochromic dye, is on the order of 0.5-1.5 mm thick offers a more robust format than a thin coating. The photochromic functionality of the layer is not lost when the surface is scratched. A deeper dye-containing layer also has the potential of sustaining viable photochromic dye molecules over the intended life of the lens when exposed to solar UV radiation. The challenge is to provide a cast composite layer system that adheres to the polycarbonate through lens processing, edging, frame assembly and normal wear. A thick layer can have more internal stress problems than a thin coating, and stress and/or adhesion problems resulting from such thick layers have often prohibited the successful marketing of photochromic polycarbonate lenses with a cast composite layer.
U.S. Pat. No. 5,219,497 (issued Jun. 15, 1993) describes a method of casting a composite layer on a variety of plastics for producing finished plastic aspheric multifocal or progressive lenses. A photochromic composition containing a UV initiator is coated onto a preformed lens in a mold, and is polymerized using UV light and heat.
U.S. Pat. No. 5,914,174 (issued Jun. 22, 1999) concerns a method of manufacturing composite photochromic lenses in which the composite layer is 25-1000 micrometers in thickness. A “precoat resin” containing the photochromic dyes is coated or formed onto the concave surface of a mold. In one embodiment, a lens body may be cast onto the mold with “precoat resin”. In another embodiment, the mold with the “precoat resin” has a polymerizable resin added to it, and a lens blank is then contacted to the mold assembly. The “precoat” resin is gelled to the point of using up all the initiator prior to the assembly process. The resin contains a mixture of at least two polymerizable components, at least one photochromic additive, at least one additive and at least one polymerization initiator. The mixture of at least two polymerizable components is preferably selected from monofunctional, difunctional and multifunctional acrylates and methacrylates.
U.S. Pat. No. 6,455,653 (issued Sep. 24, 2002) suggests the use of ethoxylated bisphenol A diacrylates in the photochromic layer of composite lenses. The photochromic compositions are coated onto a preformed lens in a mold. Polymerization of the composition is performed by a two stage process involving a first low intensity UV light step, followed by a high intensity UV light step. The composition is preferably heated to about its glass transition temperature prior to polymerization.
Another possible solution to the challenge of adhering of the composite layer to the lens body is illustrated in U.S. Pat. No. 5,316,702 (issued May 31, 1994). A method of casting the lens preform with a regular pattern of grooves to improve adhesion to the cast composite layer is described. The liquid resin and the preform interact at a contact angle of 20 to 45 degrees in order to optimize spreading of the resin on the preform.
Despite the extensive efforts already described that attempt to overcome the challenges inherent in the production of photochromic polycarbonate lenses, there still exists a need for new materials and methods for the production of these commercially important lenses.